Process for ion-supported vacuum coating

ABSTRACT

A process an device for ion-supported vacuum coating. 
     The process and the affiliated device is intended to permit the high-rate ating of large-surfaced, electrically conductive and electrically insulating substrates with electrically insulating and electrically conductive coatings with relatively low expenditure. The substrates are predominantly band-shaped, in particular plastic sheets with widths of over a meter. 
     According to the invention, in an intrinsically known device for vacuum coating, alternating negative and positive voltage pulses are applied to the electrically conductive substrate or in electrically insulating substrates, to an electrode disposed directly behind them, e.g. the cooling roller, relative to the plasma or to an electrode that is disposed almost at plasma potential. The form, the voltage, and the duration of the pulses are adapted to the coating task and the material. 
     The process is used particularly for depositing abrasion protection, corrosion protection, and barrier coatings. The user is the packaging industry, among others.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the priority under 35 U.S.C. § 119 ofGerman Patent Application No. P 44 12 906.8 filed on Apr. 14, 1994.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process and the affiliated device forion-supported vacuum coating of electrically conductive or electricallyinsulating substrates with electrically insulating coatings and ofelectrically insulating substrates with electrically conductivecoatings. A preferred application field is the ion-supported vacuumcoating of plastic sheets with electrically insulating oxide coatings.The depositing of abrasion protection coatings and corrosion protectioncoatings as well as barrier coatings for packaging purposes is ofparticular significance.

2. Discussion of Background Information

It is known that ion-supported vacuum coating can produce a higherpacking density and therefore a greater hardness and improved barrierproperties of the growing coating.

One variant of ion-supported vacuum coating is comprised in disposing anion source in the coating chamber in addition to the coating source sothat the substrate is struck by the atoms or molecules of the coatingmaterial at the same time as being struck by inert case ions or reactivegas ions. As a result of the ion bombardment, loose coating componentsare removed from the coating surface or moved to positions that are morefavorable from an energy standpoint by means of impact processes. Whendepositing coatings made of chemical compounds, e.g. oxides, bombardmentwith reactive gases, e.g. with oxygen ions, can improve thestoichiometry of the coatings in addition to the packing density. Whencoating electrically insulating substrates or when depositingelectrically insulating coatings, it is necessary to neutralize the ionbeam by adding electrons so that the coating does not becomeelectrically charged. To this end, the ion sources are equipped withsuitable neutralizing agents.

The previously available ion sources are unsuitable for ion-supportedcoating with high coating rates of the kind required for example forcoating plastic sheets or metal band in band coating devices. On the onehand, the high ion flows adapted to the high coating rates cannot beproduced at a justifiable expense. On the other hand, themaintenance-free service life of the ion sources does not correspond tothe requirements of industrial application because of the high vapordensities and the attendant high soiling rates.

It is also known to realize the ion-supported coating without usingseparate ion sources in order to prevent these disadvantages. To thisend, ions are produced between the coating source and the substrate byvarious means and are accelerated at the substrate by applying anegative bias voltage to it. The production of ions can be carried outby a self-maintained or non-self-maintained glow discharge, an arcdischarge, or--when coating by means of high-rate atomizing--a magnetrondischarge. The acceleration of ions onto the substrate by means ofapplying a negative bias voltage, though, is only possible whendepositing electrically conductive coatings on electrically conductivesubstrates (Vakuum-beschichtung Vacuum Coating!, Vol. 4, G. Kienel,VDI-Verlag Dusseldorf, 1993, p. 33).

It is known that insulating substrates or insulating coatings charge bythemselves to a slight negative bias voltage on the order of -10 V whena high-density plasma is produced in the immediate vicinity of thesubstrate surface. This so-called self bias voltage and the resultantion energies, though, are too slight to produce the desired structuralimprovement and in particular, the increased packing density of thedeposited coating.

When coating insulating substrates or when depositing insulatingcoatings, it is furthermore known to increase this self bias voltage byvirtue of the fact that a high-frequency electrical alternating field isbrought into effect perpendicular to the substrate surface. Above theso-called cutoff frequency, which is situated at a few megahertzdepending upon the geometry and alternating field amplitude, theelectrons of the plasma are excited to intensive oscillations while theions cannot follow the high frequency due to their greater mass. Theresult is an increased ionization of the plasma and a negative chargingof the substrate or coating surface, which charging depends on thegeometry and alternating field amplitude. Normally, this high-frequencyexcitation is carried out at the frequency of 13.56 MHz, which iscleared for industrial applications.

The use of this kind of high frequencies, though, is connected withconsiderable technical expense and therefore is limited to relativelysmall substrate areas for economical reasons. Typical uses are thecoating of optical components and electronic elements. The transmissionof high-frequency output requires matching networks whose loss powersharply increases with the capacity of the high-frequency electrodes,i.e. with the substrate size. When coating plastic sheets, for examplethe entire cooling roller via which the sheet is guided during coatingmust be embodied as a high-frequency electrode. With coating widths onthe order of one meter or more, the required expenditure for thehigh-frequency generator and matching network is no longer economicallyjustifiable.

Finally, ion-supported coating of substrates, which are electricallynon-conductive or poorly conductive, is also known. To that end,electrodes that are connected to alternating current are disposed in theplasma chamber between the vapor source and the substrate to be coated.The frequency of this alternating current lies between a few kHz and afew hundred kHz, wherein there is a voltage of +250 V in relation to theplasma potential of -800 V. In the process, the glow cathode constantlyand uniformly emits electrons (DD 161 137 A 3). This process and theaffiliated device have the disadvantage that the coatings produced haveonly a low packing density so that the quality of the coatings,particularly on plastic sheets, does not correspond to the high demandsthat are placed on corrosion protection coatings and barrier coatings.Furthermore, the cost of producing high frequencies is relatively high.

SUMMARY OF THE INVENTION

The object of the invention is to find a process and a device forion-supported vacuum coating, which permits the high-rate coating oflarge-surfaced electrically conductive or electrically insulatingsubstrates with electrically insulating coatings and electricallyinsulating substrates with electrically conductive coatings, with aneconomically justifiable expenditure that is as low as possible. Inparticular, plastic sheets or other band-shaped materials with a widthof over a meter can be coated at a reasonable price. The equipmentexpenditure of the device should not significantly exceed that which isotherwise normal.

Accordingly, the present invention may be directed to a process forion-supported vacuum coating, for a high-rate coating of a substrateincluding at least one of a large-surfaced, electrically conductive orelectrically insulating substrate with an electrically insulated coatingand of an electrically insulating substrate with an electricallyconductive coating, in which a plasma is produced between a coatingsource and the substrate to accelerate ions toward the substrate. Themethod may include applying alternating negative and positive voltagepulses, relative to a plasma potential, to one of the electricallyconductive substrate and an electrode disposed directly behind theelectrically insulating substrate and extending over an entire coatingsurface. A duration of the negative voltage pulses may correspond to acharging time of capacitor formed by at least one of the electricallyinsulated coating and the electrically insulating substrate, and themethod may further include choosing a duration of the positive voltagepulses to be equal to or less than the duration of the negative voltagepulses. The positive and negative voltage pulses may follow one anotherin direct succession and may be set at virtually a same level relativeto the plasma potential, and the level of the positive and negativevoltage pulses, relative to the plasma potential, may be approximatelybetween ±20 to ±2000 V.

In accordance with another feature of the present invention, the methodmay further include adjusting the duration of the negative voltagepulses to between approximately 1 ms to 10 μs when at least one of aninsulating coating thickness and substrate thickness is betweenapproximately 1 μm to 100 μm.

In accordance with still another feature of the present invention, themethod may further include applying rectangular voltage pulses to one ofthe electrically conductive substrate or the electrode behind theelectrically insulating substrate. The method may also include selectinga large rise time of the positive voltage pulses and a large fall timeof the negative voltage pulses so that surface potential of depositedcoating is not increased at any point in time more than 20 V positiverelative to the plasma potential, and selecting a small rise time of thenegative voltage pulses and a small fall time of the positive voltagepulses so that the surface potential of the deposited coating isincreased for a short time to a negative value of at least 50 percent adoubled pulse level relative to the plasma potential.

In accordance with yet another feature of the present invention, themethod may include applying sine-shaped voltage pulses to one of theelectrically conductive substrate and the electrode behind theelectrically insulating substrate, and adjusting the durations of thepositive and negative voltage pulses to be equal in length.

In accordance with still another feature of the present invention, themethod may include utilizing an electron beam evaporator as a coatingsource, and producing the plasma by ionizing vapor and residual gas withan electron beam and backscattering the electrons at the evaporatingmaterial.

In accordance with a further feature of the present invention, themethod may include utilizing at least one arbitrarily heated evaporatoras a coating source, and producing the plasma by low-voltage arcdischarges in the region between the at least one arbitrarily heatedevaporator and substrate.

In accordance with another feature of the present invention, the methodmay include utilizing at least one resistance-heated boat evaporator asthe coating source, and producing the plasma by hollow-cathode arcdischarges in the region between the at least one evaporator and thesubstrate.

In accordance with yet another feature of the present invention, themethod may include utilizing at least one arbitrarily heated evaporatoras the coating source, and producing the plasma by a magnetron dischargebetween two magnetrons which burn counter to each other with achronologically alternating polarity.

In accordance with still another feature of the present invention, themethod may include utilizing at least one atomizing source as a coatingsource, the at least one atomizing source simultaneously producing theplasma.

In accordance with a further feature of the present invention, the levelof the positive and negative voltage pulses, relative to the plasmapotential, may be approximately between ±50 to ±500 V.

In accordance with another feature of the present invention, the methodmay also include choosing the duration of the positive pulses such thatthe duration of the negative pulses are approximately 2 to 10 timesgreater than the chosen duration of the positive pulses.

The present invention may be directed to a device for ion-supportedvacuum coating and for providing a high-rate coating of a substrate, thesubstrate including one of a large-surfaced, electrically conductive orelectrically insulating substrate with an electrically insulated coatingand of an electrically insulating substrate with an electricallyconductive coating. The device may include at least one coating source,a device for securing or guiding the substrate, a device for producing aplasma, and a voltage source, producing voltage pulses, and including afirst and second pole. The first pole may be coupled to one of theelectrically conductive substrate and an electrode disposed directlybehind the electrically insulating substrate, and the second pole may becoupled for receiving substantially a plasma potential.

In accordance with another feature of the present invention, thesubstrate may further include band-shaped, electrically insulatingsubstrates, and the electrode may include a cooling roller insulatedagainst ground potential.

In accordance with still another feature of the present invention, thecoating source may include an electron beam evaporator and a plasmaconfining field positioned between the evaporating material and thesubstrate for deflecting backscattered electrons. The plasma confiningfield may include first and second plasma electrodes which aremaintained almost at the plasma potential.

In accordance with yet another feature of the present invention, thedevice may further include low-voltage arc sources, includinghollow-cathode arc sources and at least one anode, positioned betweenthe coating source and the substrate. The at least one anode may includethe second pole at substantially the plasma potential.

In accordance with a further feature of the present invention, thedevice may further include a plurality of adjacent alternatingcurrent-heated boat evaporators longitudinally positioned in a directionof substrate transport, and a hollow-cathode arc source, including ahollow-cathode arc essentially extending in a longitudinal direction ofthe plurality of alternating current-heated boat evaporator, positionedabove the plurality of alternating current-heated boat evaporators.

In accordance with a still further feature of the present invention, thedevice may also include a magnetic deflection system associated witheach hollow-cathode arc source for horizontal alternating deflection ofthe hollow-cathode arcs.

In accordance with still another feature of the present invention, thedevice may further include at least two magnetrons arranged to burncounter to each other and with polarity that alternates at apredetermined period and to be insulated against ground potential. Theat least two magnetrons may be disposed between the coating source andthe substrate to produce the plasma, and the voltage source may becoupled to both of the magnetrons.

In accordance with yet another feature of the present invention, thedevice may further include one of the at least two magnetrons coupled tothe one of the electrically conductive substrate and the electrodedisposed directly behind the electrically insulating substrate.

In accordance with another feature of the present invention, the voltagesource may include two voltage sources connected in series, operating ina synchronous manner, and working in a same direction, and may furtherinclude a voltage tap positioned between the at least two magnetrons,and coupled with the one of the electrically conductive substrate or tothe electrode disposed directly behind the electrically insulatingsubstrate.

In accordance with yet another feature of the present invention, thevoltage source may be coupled to at least two series connected electricresistors of equal size, and the one of the electrically conductivesubstrate and the electrode disposed directly behind the electricallyinsulating substrate may be coupled to a connection point between the atleast two series connected resistors.

In accordance with still another feature of the present invention, thecoating source may include at least one magnetron atomizing source, andeach at least one magnetron atomizing source may be disposed for thesimultaneous production of the plasma.

In accordance with another feature of the present invention, the atleast one the magnetron atomizing source may include one of reactivelydriven d.c. magnetrons and high-frequency magnetrons.

In accordance with yet another feature of the present invention, the atleast one the magnetron atomizing source may include two magnetronsarranged to burn counter to each other with alternating polarity and thevoltage source further for coupling to the at least two magnetronatomizing source.

The acceleration of ions from the plasma, which is produced between thecoating source and substrate, toward the insulating substrate or theinsulating coating can also be carried out without the self biasproduction by means of an expensive high-frequency field if alternatingnegative and positive voltage pulses relative to the plasma potentialare applied to the substrate in the case of conductive substrates or toan electrode disposed directly behind the substrate in the case ofinsulating substrates and the duration of the pulses is adapted to thecharging time of the capacitor, which is constituted by the insulatingcoating or the insulating substrate. The voltage pulses applied to theback side of the insulating coating or the insulating substrate aretransmitted as in a capacitor, to the surface of the insulating coatingor of the insulating substrate, which surface is oriented toward theplasma, as long as no current flow of plasma onto this surface occurs.As a result, a negative and a positive bias voltage are alternatinglyproduced on the insulating surface of the substrate or the coating,which surface is oriented toward the plasma. During the negative voltagepulse, positive ions are accelerated out of the plasma onto the surfaceof the substrate or coating and during the positive voltage pulse,electrons are accelerated out of the plasma onto this surface. As aresult of the attendant ion or electron flow, though, after a shorttime, the capacitor constituted by the insulating substrate and/or theinsulating coating becomes charged. The surface oriented toward theplasma then assumes the plasma potential and no further ions orelectrons can be withdrawn from the plasma. Because of the alternationof negative and positive voltage pulses which adjoin in directsuccession and whose length is adapted to the charging time of thecapacitor constituted by the insulating substrate and/or the insulatingcoating, a succession of directly connected ion and electron flows tothe substrate or coating surface are obtained. At the same time, it hasturned out that with plasma densities of approx. 10¹⁰ cm⁻³, the durationof negative pulses should be on the order of 1 ms for insulating coatingthicknesses or substrate thicknesses of approx. 1 μm and should be onthe order of 10 μs for insulating coating thicknesses or substratethicknesses of approx. 100 μm. The duration of positive pulses can beshorter than the duration of negative pulses because of the highermobility of electrons in comparison to ions. A duration of the positivepulses that is lesser by a factor of 2 to 10 can be sufficientlyrealized with regard to the time requirements and can be realized withrelative ease as regards the technical expenditure.

As a result, suitable pulse train frequencies of approx. 1 kHz forapprox. 1 μm thick insulating coatings on conductive substrates or ofapprox. 100 kHz for insulating or conductive coatings on insulatingsubstrates that are approx. 100 μm thick. In contrast to the knownhigh-frequency excitation at 13.56 MHz, frequencies of this kind can beproduced with much less expensive generators and without costlyadapters. Another advantage of the process according to the invention,with alternating positive and negative bias voltage in the middlefrequency range in comparison to the continuous, negative self biasvoltage with high-frequency excitation is comprised in that voltagepunctures and micro-arcs that can occur for bias voltages greater than1000 V, in particular with thin insulating coatings or thin insulatingsubstrates, do not occur or drop into the non-critical range because ofthe polarity reversal in the middle frequency range.

It is suitable for the carrying out of the process according to theinvention to use rectangular voltage pulses, wherein the rise times andfall times of the pulses are intended to be optimally adapted to theprocess by means of corresponding wiring. The fall time of the negativepulses and the rise time of the positive pulses connected to them shouldbe selected of such a size that the surface potential of the insulatingcoating is not increased at any point in time by more than 20 V positivein comparison to the plasma potential. This is achieved if the positivevoltage increase is adjusted at a speed less than or equal to thenegative charging of the insulating coating surface by electrons fromthe plasma. Because of this measure, it is achieved that the electronsare accelerated with as low as possible a bias voltage and the substrateis not unnecessarily loaded with additional energy. On the other hand,the fall time of the positive pulses and the rise time of the negativepulses connected to them should be as short as possible so that thedesired negative bias voltage is achieved on the substrate or coatingsurface and is not ever reduced during the negative voltage increase bymeans of the ion flow from the plasma. The higher the particle densityof the plasma and therefore the higher the ion flow onto the substrateor coating surface, the more rapidly the negative voltage increase mustoccur.

In applications with lower plasma density, a slower negative voltageincrease is permissible and instead of the rectangular voltage pulses,sine-shaped voltage pulses are used which are cheaper to produce. In thecourse of this, the frequency must be chosen as high so thatsufficiently high negative bias pulses are achieved on the substrate orcoating surface.

A particularly advantageous use of the process according to theinvention is comprised in the ion-supported coating of conductivesubstrates with thin insulating coatings. Thin insulating coatings up tothe thickness of a few μm constitute a capacitor, which has highcapacity and high charging time, between the conductive substrate andthe plasma and therefore require only relatively small and inexpensiveto produce pulse train frequencies. Furthermore, with ion-supportedcoating with high-frequency excitation, thin insulating coatings arevery prone to voltage punctures and micro-light arcs so that the use ofthe process according to the invention represents almost the onlypossibility for a stable and low-defect ion-supported coating.

Another advantageous use of the process ensues in the ion-supportedcoating of thin plastic sheets with insulating or conductive coatings.Plastic sheets coated in this way are required for example as barriersheets in the packaging industry and require minimal manufacture costs.The thicknesses of plastic sheets that are of interest in thisconnection are 10-20 μm so that even in this case, the process accordingto the invention can be used with relatively low pulse trainfrequencies.

A particularly favorable embodiment of the process according to theinvention ensues if an electron beam evaporator is used as the coatingsource. In this case, a plasma is already produced by the electron beamand by backscattered electrons, and a separate plasma source is oftenunnecessary.

However, higher plasma densities are required for depositingparticularly high-quality coatings or for the achievement ofparticularly high coating rates. In this case, it is suitable to producethe requisite high plasma densities by means of low voltage arcdischarges, in particular by means of hollow-cathode arc discharges. Incontrast, if a high homogeneity of the coating must be achieved overlarge coating widths, then it is advantageous to produce the plasma bymeans of an intrinsically known magnetron discharge between two pulsemagnetrons that burn counter to each other with polarity thatchronologically alternates.

A particularly advantageous solution ensues if the coating occurs byitself or in addition to the atomization by means of one or a number ofatomizing sources. In this case, a separate plasma source is notrequired and through the combination of the coating by atomization withthe alternating middle frequency pulse bias according to the invention,particularly high-quality coatings are obtained. Reactively operatedd.c. magnetrons or high-frequency magnetrons are regarded as atomizingsources. Particularly low-defect coatings are obtained with the use ofthe above-mentioned pulse magnetron as a coating source.

The most suitable device for carrying out the process according to theinvention is an intrinsically known vacuum coating device that isequipped with a coating source, a substrate retainer or guide device,and means for producing a plasma. Additionally, a voltage source isrequired for producing voltage pulses of alternating polarity with pulselevels between ±20 V and ±2000 V and pulse durations between 1 μs and1000 μs. One pole of this voltage source is connected to an electrodethat is disposed approximately at plasma potential; in the case ofconductive substrates, the other pole is connected to the substrate,which is insulated from the plasma, and in the case of an insulatingsubstrate, this other pole is connected to an electrode that is disposeddirectly behind the substrate and extends over the entire coating area.

With the coating of thin insulating plastic sheets, the cooling rollerthat is usually provided to guide and cool the plastic sheet in thecoating zone can be used as an electrode disposed directly behind thesubstrate. However, then this cooling roller, in contrast toconventional sheet coating devices, must be insulated against groundpotential and if need be, must be provided with shields that guardagainst the ignition of parasitic glow discharges.

In devices with electron beam evaporators as the coating source, it isuseful to dispose an intrinsically known plasma confining field betweenthe evaporating material and the substrate. As a result, thebackscattered electrons on the evaporating material are forced intocircular paths so that their path and therefore their ionizationprobability in vapor and residual gas is increased and also a plasmadensity that is sufficient for the process according to the invention isachieved without an additional plasma source.

For the production of particularly high ion flow densities on thesubstrate, it is useful to dispose low-voltage arc sources as additionalplasma sources between the coating source and the substrate.Hollow-cathode arc sources are particularly suitable. When usingalternating current-heated evaporator boats as a coating source inconnection with hollow-cathode arc sources, a lateral, alternatingdeflection of the hollow-cathode arcs by means of the magnetic strayfields of the alternating current-heated evaporator boats turns out tobe a particular advantage. This is especially true when using anintrinsically known magnetic guide field parallel to the hollow-cathodearcs. As a result, a higher uniformity of the plasma is achievedcrosswise to the hollow-cathode arcs.

Another increase in uniformity, particularly when using only onehollow-cathode arc source for a number of boat evaporators, is achievedby disposing intrinsically known magnetic deflection systems on thehollow-cathode arc sources, which systems carry out an additionalhorizontal, alternating deflection of the hollow-cathode arcs.

Another embodiment of the device according to the invention ensues withthe use of two pulse magnetrons, which have laterally alternatingpolarity, as plasma sources. In this case, the pulse voltage source forthe two pulse magnetrons can be used at the same time as a voltagesource for the pulse bias voltage according to the invention. Thesimplest solution is comprised in producing an electrical connectionbetween one of the two pulse magnetrons and the substrate or theelectrode disposed directly behind the substrate. Since the plasmapotential changes periodically relative to each of the two pulsemagnetrons in accordance with the applied pulse voltage, the sameperiodic pulse voltage is also obtained between the plasma and thesubstrate. If the magnetron connected to the substrate receives apositive pulse, then electrons are accelerated from the plasma to thesubstrate. If it receives a negative pulse, then ions are accelerated tothe substrate.

Another variant is comprised in that the pulse voltage source iscomprised of two voltage sources, which are connected in series, operatein a synchronous manner, and work in the same direction, and that avoltage tap is provided between these two pulse generators, which isconnected to the substrate or to the electrode disposed directly behindthe substrate. In comparison to the variant mentioned at the beginning,in this case, with the same magnetron pulse voltage, only half the pulsevoltage is obtained between the plasma and substrate. Whenever themaximum of the positive pulse voltage is achieved at one of the twomagnetrons, ions are accelerated from the plasma to the substrate.During the pole reversal of the two magnetrons, the plasma potentialshifts in the negative direction for a short time so that an electroncurrent flows for a short time onto the substrate. A similar variantensues if only one pulse voltage source is provided for the two pulsemagnetrons and the two outputs of the pulse voltage source are connectedvia electric resistors of equal size to the substrate or the electrodedisposed directly behind the substrate. In this case, the same potentialratios are obtained as in the above-mentioned case, but only one pulsevoltage source is required in all.

A similar embodiment of the device according to the invention ensueswith the use of atomizing sources as the sole coating source or inaddition to arbitrarily heated evaporator sources. In this case, theatomizing sources are simultaneously used as a coating source and aplasma source. With the use of d.c. magnetrons or high-frequencymagnetrons as atomizing sources, separate voltage sources are used forthe operation of the magnetron and the production of the voltage pulsesbetween the plasma and the substrate. In the course of this, the targetsof the magnetron, which are electrically connected to one another, areused as electrodes disposed almost at plasma potential. With the use ofpulse magnetrons as atomizing sources, the pulse voltage source for thepulse magnetron can be used simultaneously as the pulse voltage sourcefor producing the voltage pulses between the plasma and the substrate.At the same time, the same variants can be used which were described asthe plasma source in the use of pulse magnetrons.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in detail in conjunction with severalexemplary embodiments. In the accompanying drawings,

FIG. 1 shows schematic representation of a device for carrying out theprocess,

FIG. 2 shows a chronological course of the potentials and currents asthe process is carried out,

FIG. 3 shows a device for coating plastic sheets, having an electronbeam evaporator as a coating source and a hollow-cathode arc source as aplasma source,

FIG. 4 shows a top view of a device for coating plastic sheets, havingboat evaporators as a coating source and hollow-cathode arc sources asplasma sources,

FIG. 5 shows a device for coating plastic sheets, having boatevaporators as a coating source and pulse magnetrons as a plasma source,

FIG. 5a shows an embodiment of the device according to FIG. 5, with twovoltage sources,

FIG. 5b shows an embodiment of the device according to FIG. 5, with avoltage divider, and

FIG. 6 shows a device for coating plastic sheets, having a plasma sourceand two pulse magnetrons as a coating source.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a device for carrying out the process, having a coatingsource 1, a plasma source 2, the plasma 3 produced, an insulatingcoating 4, and a conductive substrate 5. A capacitor is embodied as adialectric via the insulating coating 4. The upper end of the insulatingcoating 4 charges up to the potential U_(S) via the conductive substrate5 and the lower end of the insulating coating 4 charges up to thepotential U_(B) via the plasma 3. In the process, the current I flowsfrom the plasma 3 onto the underside of the insulating coating 4.Because of the direct contact between the conductive substrate 5 and theinsulating coating 4, U_(S) agrees with the potential of the conductivesubstrate 5, while the potential U_(B) differs from the potential U_(P)of the plasma on the underside of the insulating coating 4.

In principle, FIG. 2 shows the chronological course of potentials U_(S)and U_(B) relative to the plasma potential U_(P) as well as the courseof the current I if during the time t₊, a positive voltage is applied tothe conductive substrate 5 and during the time t₋, a negative voltage isapplied, which voltage applications occur in an alternating fashion. Thepotential U_(S) directly follows the voltage applied to the conductivesubstrate 5. In contrast, due to the capacitor function of theinsulating coating 4, the potential U_(B) actually follows the rapidvoltage jumps, but then as a result of the charge carrying current I_(B)from the plasma 3, falls very rapidly until almost at the plasmapotential. This drop of the potential U_(B) occurs more rapidly thehigher the current I from the plasma is. After negative voltage jumps, acurrent I_(i) of positive ions is drawn from the plasma 3 onto thesurface of the insulating coating 4, and after positive voltage jumps, acurrent I_(e) of negative electrons is drawn from the plasma 3 onto thissurface. Due to the higher mobility of electrons, the negative electroncurrent I_(e) is considerably greater than the positive ion currentI_(i). Accordingly, the time t_(e) for charging the capacitor by meansof the electron current I_(e) is shorter than the time t_(i) forcharging by means of the ion current I_(i). Since in the chronologicalmiddle, no charge of the insulating surface can be discharged, then thefollowing equation is true:

    I.sub.i ·t.sub.i =I.sub.e ·t.sub.e.

In order to shoot the greatest possible number of ions onto the surfaceof the insulating coating 4, it is suitable to adapt the time durationst₋ and t₊ of the voltage pulses to the charging times t_(i) and t_(e) ofthe capacitor constituted by the insulating coating 4:

    t.sub.- ≈t.sub.i ; t.sub.+ ≈t.sub.e

Furthermore, it is useful that the negative voltage changes of U_(S)(shown in FIG. 2 as a dashed line) occur much faster than the chargetime t_(i) of the insulating coating 4 by means of positive ions andthat the positive voltage changes of U_(S) occur at a speed less than orequal to the charging time t_(e) of the insulating coating 4 by means ofelectrons. As a result, it is achieved that the level of the negativevoltage pulses U_(B) on the underside of the insulating coating 4, whichis used as a bias voltage for the ions extracted from the plasma 3 isonly reduced by an insignificant amount. In contrast, the relativelyslow increase of the positive pulses leads to the fact that the voltagedrop occurs at greater than or equal to the speed of the voltageincrease influenced by the conductive substrate 5. The result is thatthe positive potential U_(B) on the underside of the insulating coating4 is not significantly increased over the plasma potential U_(P) so thatthe electrons withdrawn from the plasma 3 do not transmit anyimpermissibly high energy to the coating 4 and the substrate 5. This isparticularly important in the coating of temperature sensitivesubstrates 5 such as plastic sheets.

FIG. 3 shows a device for carrying out the process for coating plasticsheets by means of ion-supported electron beam evaporation of aluminumoxide. A 10 μm thick, electrically insulating plastic sheet 6 (polyestersheet) is vacuum coated with a 0.05 μm thick, electrically insulatingoxide coating 7, while it is conveyed in a known manner in the vacuumvia a cooling roller 8 cooled to approx. -20° C. The aluminum oxide 9 isevaporated from a water-cooled crucible 10 by means of an electron beam12 produced in an electron cannon 11. The electrons backscattered on thealuminum oxide 9 are deflected by means of a magnetic field 14 producedin a plasma confining field 13 and in this way, are prevented fromstriking and heating up the oxide coating 7 and the plastic sheet 6.Because of the ionizing effect of these backscattered electrons, aplasma 15 of lower density is produced between the crucible 10 and theplastic sheet 6 to be vacuum coated. For the production of higher plasmadensities, a cathode 16 of a hollow-cathode arc source is disposed abovethe plasma confining field 13. The low-voltage electron beam 17 thusproduced is conveyed by means of the boundary field lines of themagnetic field 14 to the anode 18 of the hollow-cathode arc source. Thevapor stream 19 of the evaporated aluminum oxide and the oxygen admittedvia a gas admission system 20 are excited, ionized, and dissociated bymeans of the low-voltage electron beam 17 so that a high-density plasma21 is produced beneath the plastic sheet 6 to be vacuum coated. Betweenthe anode 18, which is disposed virtually at plasma potential, and thecooling roller 8, which is insulated against ground potential and actsas an electrode directly behind the insulating plastic sheet 8, avoltage source 22 is disposed for producing rectangular voltage pulsesof alternating polarity. During the negative pulses at a level ofapprox. 200 V and approx. 20 μs duration, ions from the high-densityplasma 21 are accelerated onto the plastic sheet 6 or the growing oxidecoating 7 in addition to the likewise accelerated, partially ionized andexcited vapor and oxygen atoms in the plasma. During the directlysucceeding positive pulse likewise at a level of approx 200 V, but onlyapprox. 5 μs duration, electrons are accelerated from the high-densityplasma 21 onto the plastic sheet 6 or the oxide coating 7. The aluminumoxide coating deposited in this manner has a high optical transparencyand a high packing density and is particularly suited as a barriercoating for high-quality packaging sheets.

FIG. 4 shows a top view of the section of a device for carrying out theprocess for the ion-supported coating of plastic sheets with aluminumoxide by means of reactive evaporation of aluminum. A series ofalternating current-heated boat evaporators 23 disposed next to oneanother, which are aligned in the transport direction of the plasticsheet 6, are used as the coating source. (Approximately 15 boatevaporators are disposed next to one another for coating 1.50 m widesheets.) Hollow-cathode arc sources, which are comprised of hollowcathodes 16 and a common anode 18, are in turn disposed between the boatevaporators 23 and the plastic sheet 6 for the production of ahigh-density plasma 21. The low-voltage electron beams 17 produced atthe hollow cathodes 16 are conveyed by means of a magnetic field 24running in the transport direction of the plastic sheet 6 and aredeflected lateral to the transport direction of the plastic sheet 6 bythe deflection system 25 in the vicinity of the hollow cathodes 16 inorder to produce an even distribution of the plasma over the entirewidth of the plastic sheet 6. In addition, to heat the boat evaporators23, the alternating currents produce annular magnetic fields 26, whichrun around the boat evaporators 23 and superpose the field intensity ofthe magnetic field 24 and as a result, lead to an additional lateraldeflection of the low-voltage electron beams 17 and consequently to afurther homogenizing of the plasma 21. Analogous to FIG. 3, voltagepulses of alternating polarity are applied between the anode 18, whichis disposed approx. at plasma potential, and the cooling roller 8, whichis not shown, but is disposed above the coating source to convey theplastic sheet 6, likewise analogous to FIG. 3, in order to alternatinglyshoot ions and electrons from the plasma onto the growing coating 7.

FIG. 5 shows another device for carrying out the process, likewise withboat evaporators 23 as a coating source, but with two magnetrons 27, 28driven in pulse fashion to produce plasma 21 for the ion-supportedcoating. Negative potential (cathode) is applied to one magnetron 27 andpositive potential (anode) is applied to the other magnetron 28,respectively, with chronologically alternating polarity by means of thevoltage source 22. One of the two magnetrons 27; 28, for example themagnetron 28, is connected to the cooling roller 8, which is insulatedagainst ground potential and disposed behind the plastic sheet 6. As aresult, when there is negative potential of magnetron 28 in comparisonto magnetron 27, ions are accelerated from the plasma 21 onto thegrowing coating 7 on the plastic sheet 6 and when there is positivepotential of the magnetron 28 in comparison to magnetron 27, electronsare accelerated from the plasma 21 onto this coating, in achronologically alternating fashion. In this manner, with operation ofboth magnetrons 27, 28 in inert gas with no percentage of reactive gas,a pure metal coating with a high packing density and a low electricalresistance is deposited. With the aid of the wire feeding device 29 andwith the use of correspondingly suitable boat evaporators 23, virtuallyall metals that are available in wire form, including metal alloys, canbe vaporized. With the additional admission of a reactive gas, e.g.oxygen, via the gas admission system 20, the corresponding metalcompounds, e.g. metal oxides, can be deposited. As a result of theactivation of the vapor stream 19 and the admitted reactive gas, whentraveling through the plasma 21, stoichiometric coatings are deposited,even at high coating rates. Furthermore, by means of the ion bombardmentthat is carried out in pulse fashion, a particularly dense andlow-defect coating can be produced, which is suited as a barrier coatingfor packaging sheets or as a corrosion or abrasion protection coatingfor the substrate or for a coating already deposited on the substrate atan earlier point. Furthermore, because of the high uniformity of theplasma 21 produced by the magnetron 27, 28 and the virtually arbitrarylength of the magnetron 27, 28, highly constant coating properties canalso be produced over coating widths of a number of meters.

FIG. 5a shows a variant of the device shown in FIG. 5, in which twovoltage sources 22a and 22b, which are connected in series, operate in asynchronous manner, and work in the same direction, are used for theexcitation of the two magnetrons 27, 28. In this case, the voltage tapfor the electrically conductive substrate or the cooling roller 8, whichis insulated against ground potential, occurs at the connecting pointbetween the two voltage sources 22a and 22b. The advantage over thedevice according to FIG. 5 is comprised in that during each dischargingdirection of the two magnetrons 27, 28, in one, ions are acceleratedfrom the plasma onto the plastic sheet 6 and in the other, electrons areaccelerated onto this sheet, while with the device according to FIG. 5,in one discharging direction, only ions are withdrawn and in the otherdischarge direction, only electrons are withdrawn. However, with thesame pulse voltage between the magnetrons 27, 28, only half of the pulsevoltage is available between the plastic sheet 6 and the plasma.

The same properties are achieved with the variant according to FIG. 5b,as are achieved with the variant according to FIG. 5a. By means of avoltage divider, which is comprised of two electrical resistors 30a and30b of equal size, and via the voltage source 22, whose center tap isconnected to the substrate or to the cooling roller 8 used according toFIG. 5, only one voltage source 22 is required in comparison to thevariant according to FIG. 5a. However, depending upon the pulse biascurrent required for the voltage divider, an additional current isrequired from the resistors 30a and 30b so that in comparison to thevariants according to FIGS. 5 and 5a, a voltage source 22 with highercurrent efficiency is required.

FIG. 6 shows a device for carrying out the process by using sputtersources as the coating source. Of the great number of possible variantswith regard to the type and number of sputter sources, the coating isrepresented by means of two magnetron atomizing sources 31; 32. Inparticular for depositing highly insulating, low-defect coatings,superposing pulse sputtering over reactive d.c. sputtering orhigh-frequency sputtering is known. A further improvement of the coatingproperties, in particular the packing density of coatings, is possibleby means of additional use of the process according to the invention.This is particularly advantageous because not only the voltage source22a; 22b of the magnetron atomizing sources 31; 32, but also the plasma,which is produced anyway with sputtering, can be used to carry out theprocess. The coupling of the substrate or the cooling roller 8 used forthe coating of plastic sheets 6 to the voltage source 22a; 22bcorresponds to the coupling as was shown in FIG. 5a for the use ofmagnetrons as the plasma source. The variant shown in FIG. 5b wouldlikewise be usable, while the variant shown in FIG. 5 is less favorablysuited for this due to the asymmetry of the two magnetrons.

What is claimed is:
 1. A process for ion-supported vacuum coating, for ahigh-rate coating of a substrate comprising at least one of alarge-surfaced, electrically conductive or electrically insulatingsubstrate with an electrically insulating coating and of an electricallyinsulating substrate with an electrically conductive coating, in which aplasma is produced between a coating source and the substrate toaccelerate ions toward the substrate, said method comprising:applyingalternating negative and positive voltage pulses relative to a plasmapotential to one of an electrically conductive substrate or to anelectrode disposed directly behind an electrically insulating substrateand extending over an entire substrate, wherein a duration of thenegative voltage pulses corresponds to a charging time of capacitorformed by at least one of the electrically insulating coating and theelectrically insulating substrate; choosing a duration of the positivevoltage pulses to be equal to or less than the duration of the negativevoltage pulses, wherein the positive and negative voltage pulses followone another in direct succession and are set at virtually a same levelrelative to the plasma potential, and wherein the level of the positiveand negative voltage pulses, relative to the plasma potential, isapproximately between ±20 to ±2000 V.
 2. The process according to claim1, further comprising adjusting the duration of the negative voltagepulses to between approximately 1 ms to 10 μs when at least one of aninsulating coating thickness and substrate thickness is betweenapproximately 1 μm to 100 μm.
 3. The process according to claim 1,further comprising applying rectangular voltage pulses to one of theelectrically conductive substrate or the electrode behind theelectrically insulating substrate;selecting a large rise time of thepositive voltage pulses and a large fall time of the negative voltagepulses so that surface potential of deposited coating is not increasedat any point in time more than 20 V positive relative to the plasmapotential; and selecting a small rise time of the negative voltagepulses and a small fall time of the positive voltage pulses so that thesurface potential of the deposited coating is increased for a short timeto a negative value of at least 50 percent a doubled pulse levelrelative to the plasma potential.
 4. The process according to claim 1,further comprising applying sine-shaped voltage pulses to one of theelectrically conductive substrate and the electrode behind theelectrically insulating substrate and adjusting the durations of thepositive and negative voltage pulses to be equal in length.
 5. Theprocess according to claim 1, further comprising utilizing an electronbeam evaporator as a coating source, and producing the plasma byionizing vapor and residual gas with an electron beam and electronsbackscattered at the evaporating material.
 6. The process according toclaim 1, further comprising utilizing at least one arbitrarily heatedevaporator as a coating source, and producing the plasma by low-voltagearc discharges in the region between the at least one arbitrarily heatedevaporator and substrate.
 7. The process according to claim 6, furthercomprising utilizing at least one resistance-heated boat evaporator asthe coating source, and producing the plasma by hollow-cathode arcdischarges in the region between the at least one evaporator and thesubstrate.
 8. The process according to claim 1, further comprisingutilizing at least one arbitrarily heated evaporator as the coatingsource, and producing the plasma by a magnetron discharge between twomagnetrons which burn counter to each other with a chronologicallyalternating polarity.
 9. The process according to claim 1, furthercomprising utilizing at least one sputtering source as a coating source,the at least one sputtering source simultaneously producing the plasma.10. The method according to claim 1, the level of the positive andnegative voltage pulses, relative to the plasma potential, isapproximately between ±50 to ±500 V.
 11. The method according to claim1, choosing the duration of the positive voltage pulses such that theduration of the negative voltage pulses are between approximately 2 to10 times greater than the chosen duration positive voltage pulses.